Mexican prickly poppy (Argemone mexicana), also known as goatweed, Mexican thistle, prickly poppy, and yellow thistle, is a member of the poppy family (Figure 1). It is an annual or biennial plant that is found east of the Rocky Mountains, Hawaii, and Puerto Rico. In Florida, it is typically recognized as 'some type of thistle' until it flowers.

Figure 1.

Mexican prickly poppy in a bahiagrass pasture.

[Click thumbnail to enlarge.]

Young Mexican prickly poppies are often mistaken for thistles because their toothed and prickly leaf margins give them a thistle-like appearance (Figure 2). One recognizable difference is that Mexican prickly poppy exudes a yellowish milky sap when its stem is broken, while thistles do not. Another difference is that Mexican prickly poppy flowers are relatively large and yellow with 4 to 6 petals (Figure 3), while thistles have inflorescences composed of many small flowers. Like thistles, Mexican prickly poppy reproduces only through seed production. Its seeds are enclosed in a relatively large spiny capsule (Figure 4). Approximately 400 seeds can be produced in one capsule.

Key Characteristics

Leaves: Leaves can be up to 8 inches long, and are silvery-green with white veins and deep regular lobes. The upper surface of the leaf is smooth, while the underside has a few prickles along the midrib. The edges of the leaf are often lined with many prickles.

Flowers: Flowers are yellow and approximately 2.5 inches in diameter.

Seed: Seeds are produced inside a prickly capsule measuring about 1.5 inches in length. Approximately 3 to 6 openings in the capsule allow the seeds to disperse, but many seeds can remain inside the capsule for weeks until wind or animals shake the plant. Up to 400 seeds can be enclosed by a single capsule and they may stay dormant in the soil for many years.

Mexican prickly poppy is poisonous to livestock, but is not readily eaten, so there are relatively few cases of poisoning from this plant. However, plants in hay have caused poisoning. Seeds are its most toxic part, but the entire plant contains toxic elements

Although this weed is common throughout Florida, it is typically not a wide-spread problem. For those who wish to control it, 2,4-D or WeedMaster (or other products containing 2,4-D + dicamba) are the most effective and economical herbicides for control. The application rate for each herbicide is 3 pt/acre for broadcast applications. If spot spraying is necessary, a 3% solution of either 2,4-D or 2,4-D + dicamba in water is appropriate. Remember, it is best to treat a younger plant, especially before seed set occurs to ensure that seeds are not added to the soil seedbank.

Footnotes

1.

This document is SS AGR 304, one of a series of the Agronomy Department, UF/IFAS Extension. Original publication date May 2008. Reviewed March 2011. Revised February 2014. Visit the EDIS website at http://edis.ifas.ufl.edu.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication does not signify our approval to the exclusion of other products of suitable composition. All chemicals should be used in accordance with directions on the manufacturer's label.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.